Causes of Salmon Poisoning Disease

Neorickettsia helminthoeca are the bacteria that ultimately are responsible for causing salmon poisoning disease in dogs. How dogs actually become infected with these parasites is complicated, but the process is fascinating. Tiny N. helminthoeca bacteria live naturally inside of larger organisms known as Nanophyteus salmincola. These are parasitic worms, or trematodes, more commonly called “flukes.” Flukes are thick, fleshy, flat, leaf-like creatures that take in their nourishment through one or more suckers that they attach to the inside of their host animals. Nanophyteus salmincola flukes are the carriers, or vectors, of the bacterial parasites that cause salmon poisoning disease in our domestic dogs.

How dogs get salmon poisoning is even a bit more complicated. The flukes that carry the infective bacteria have their own intermediate host, which is a snail called Oxytrema silicula. “Intermediate hosts” are animals that other organisms, usually parasites, live inside of and go through some transitional stage of development while they are living inside of their host. For example, the parasite may hatch from an egg into larvae in its intermediate host. The intermediate host may or may not also act as the vector that transmits the parasite to its ultimate victim. In their capacity as intermediate hosts for Nanophyteus salmincola flukes, Oxytrema silicula snails simply carry the flukes passively while they mature. The actual bacterial infection is spread directly from infected flukes to fish to dogs. However, the snail intermediates are an essential part of the life cycle of the flukes that transmit the parasites causing salmon poisoning to domestic dogs.

The natural habitat for these particular snails is the northern Pacific rim of the United States and coastal western Canada. That is why salmon poisoning is almost always only diagnosed in dogs in British Columbia, Oregon, Washington and Northern California, and occasionally in other areas where fish infected by flukes that carry the bacteria migrate or are transported. The infected flukes leave their snail intermediate hosts and live for a short time in water in their larval stage, where they eventually come into contact with salmon or other aquatic creatures. They get inside of those creatures by burrowing through their skin, tending to lodge in their kidneys.

Dogs become infected when they eat raw freshwater or ocean salmon, trout, steelhead, giant Pacific salamanders or other fish that contain the larval stage of infected Nanophyteus salmincola flukes. Once the flukes get into a dog’s gastrointestinal tract, they mature and invade its small intestine and lymph nodes. There, they latch on and feed, causing a significant inflammatory reaction. The fluke infection itself usually doesn’t cause any measurable disease. However, during this process, the flukes release the infective Neorickettsia helminthoeca bacteria into the dog’s intestinal tissues and bloodstream, where they start to replicate. It only takes about one to two weeks for the dog to become sick. The disease usually becomes systemic, or body-wide, in short order. If not treated, salmon poisoning disease is almost always fatal in domestic dogs.

The life cycle of the organisms that cause salmon poisoning disease is maintained when the eggs of infected flukes are passed in the feces of infected dogs. The intermediate snail hosts become infected from those hatching eggs free in the environment. The infected fluke larvae eventually leave the snail and penetrate the skin of the salmon, trout or salamander. Dogs eat these fish. Then, the cycle begins again.

Preventing Salmon Poisoning Disease

Dogs living in or visiting the Pacific northwestern United States or western coastal Canada should not be permitted to eat raw fish - especially live, dead or dying salmon, steelhead, trout or similar freshwater fish from rivers, streams or lakes. If they do, their owners should take them to the closest veterinary clinic for treatment as quickly as possible. Salmon poisoning can be fatal in a very short time. Fish that are cooked thoroughly or frozen for more than 24 hours are usually not infective, because the flukes and infective bacteria can be killed by temperature extremes.

Special Notes

Salmon poisoning disease is an important illness in dogs living in or traveling to the Pacific coast of Canada or to the northwestern United States. Infected fish are found from the San Francisco area to the southern coast of Alaska, in the Pacific Ocean. They are most common from northern California to Washington’s Puget Sound, and in rivers coming from those areas that support fish migration and spawning. Infected dogs need to be treated quickly and aggressively to prevent them from dying from this disease.